The present invention relates to a sensor for measuring exhaust gas components such as nitrogen oxides and hydrocarbons produced in a combustion process and a sensing technology using the sensor. Further, the present invention relates to exhaust gas component measurement in industrial fields requiring combustion control such as a waste incinerating facility and, more particularly to a system technology for measuring an exhaust gas concentration under a high temperature atmosphere such as a vehicle and controlling combustion state based on the measured result.
There area various kinds of exhaust gas component sensors. The exhaust gas components here mean gas components other than the main components of atmosphere of nitrogen and oxygen, and gas components contained in trace amounts in an exhaust gas. The exhaust gas component sensor means a sensor for the measurement of the exhaust gases. Typical sensors can be roughly classified into semiconductor sensors and solid electrolyte sensors. The semiconductor sensor utilizes the phenomenon that concentration of conductive carriers changes when a gas to be measured is chemically adsorbed to a semiconductor element (sensing element). The change in concentration of a measured gas can be measured by converting to change in electric resistance of the semiconductor element. Nitrogen oxide (NOX) sensors used for the deoxidizing denitration process of an incinerator have been studied, and the operating temperature is in a medium-low temperature range of 200 to 400xc2x0 C. On the other hand, the solid electrolyte sensor can stably operate at a temperature above 500 to 600xc2x0 C. Therefore, the solid electrolyte sensor becomes a focus of attention as a gas sensor usable under a high temperature exhaust gas atmosphere, for example, usable for a vehicle.
An oxygen sensor using zirconia partially stabilized by adding yttria (YSZ) as a solid electrolyte is already used as a vehicle mounted sensor, and an exhaust gas sensor similarly using the YSZ as a main component is proposed. An NOX sensor is described in detail in Japanese Patent Application Laid-Open No.10-142194, and a hydrocarbon (HC) sensor is described in Japanese Patent Application Laid-Open No.10-19843. Structure of the sensing portion of these exhaust gas component sensors is formed by performing necessary shaping of a green sheet of the solid electrolyte, laminating the shaped green sheets together with platinum electrodes, and then baking the laminated structure. Since the sensor has the structure of laminating a plurality of layers including structures of a measurement chamber and a gas passage together with the electrodes, the sensor can be called as a multilayer sensor.
On the other hand, in regard to the multilayer exhaust gas component sensor, an example of employing a porous solid electrolyte layer having gas permeability is disclosed in Japanese Patent Application Laid-Open No.7-260741. In this exhaust gas component sensor, the measurement chamber and the gas passage are eliminated, and the sensor is formed by using one porous solid electrolyte layer. In order to improve sensing accuracy, a protective layer having a gas selectivity is provided in addition to general electrode protective layers.
Both of the multilayer exhaust gas component sensors described in Japanese Patent Application Laid-Open No.10-142194 and in Japanese Patent Application Laid-Open No.10-19843 are suitable for a small sized and high performance sensor. However, they have a problem in that cracks are apt to occur at baking or during being used under a heating condition. It was found by classifying fracture modes that the cracks were produced more often by other causes different from the inter-layer separation or delamination. Occurrence of the cracks needs some starting points. The starting point may be a portion where different materials are in contact with each other or a portion where thermal and/or intrinsic stress is concentrated. In the case of the multilayer sensor, a gap portion of the measurement chamber provided inside the layer or a boundary between different materials is apt to become the starting point. Therefore, in order to suppress occurrence of the cracks in the conventional sensor having the above-mentioned structure, careful attentions on homogeneity of the materials and on temperature control and so on are required at manufacturing the sensors. In addition to this, the multilayer exhaust gas component sensor has a problem in that there is limitation in simplification of the manufacturing process because it has various kinds of structures inside one layer. In more detail, in the conventional structure, the whole laminated body is difficult to be formed through a printing method because the gap portion needs to be formed in the structure.
Further, although the multilayer exhaust gas component sensor disclosed in Japanese Patent Application Laid-Open No.7-260741 is simple in structure and little in occurrence of the cracks, it has a problem in that sufficient sensing accuracy can not be obtained. An amount of the exhaust gas components is very small compared to an amount of nitrogen and oxygen. Taking an example of the NOX concentration in a vehicle exhaust gas, the NOX concentration is 0.05% or less compared to 20% of the concentration of oxygen which can be an interference gas. In order to improve the sensing accuracy, it is necessary to remove oxygen which exists in bulk and has an ill effect on measurement, and then sensing the very small amount of the exhaust gas. In the multilayer exhaust gas component sensor formed by using one porous solid electrolyte layer disclosed in Japanese Patent Application Laid-Open No.7-260741, there is a problem in that a concentration of oxygen to be removed can not be quantitatively and accurately measured. The measuring error in the exhaust gas components is affected by component of oxygen which can not be accurately removed. Further, the sensor of Japanese Patent Application Laid-Open No.7-260741 has a problem in that it is difficult to measure a very small amount of exhaust gas components because it can not perform two-step measurement, that is, removing of oxygen and then measuring the exhaust gas components.
Furthermore, when a plurality of solid electrolyte layers are laminated up to a certain amount of thickness so as to provide many functions, there occurs a problem that high response is difficult to be obtained because the gas to be measured is difficult to pass through the porous solid electrolyte layers. Quick heat-up of the sensing part is also difficult to be realized.
In regard to combustion control in a vehicle, the system has an oxygen sensor and an air-to-fuel ratio sensor, but does not have any on-board sensor capable of accurately and stably measuring exhaust gas components. Therefore, it is impossible to directly measure nitrogen oxide and hydrocarbon in order to reflect the combustion control.
An object of the present invention is to provide a multilayer gas component measuring apparatus having a good responsive characteristic with high reliability and easy fabrication.
According to the present invention, since there is no measurement chamber and no gas passage in each of laminated layers, number of the starting points of cracks under heating condition at manufacturing or using the sensor can be reduced and as a result occurrence of the cracks can be suppressed. Quick heat-up of the sensor is also abstained.
Further, since there is no structures such as a measurement chamber and a gas passage in each of laminated layers, the laminated body is easily formed by laminating the green sheets and/or through a printing method.
Further, by using a plurality of solid electrolyte layers including gas-penetrative solid electrolyte layers, it is possible to accurately remove oxygen component and to accurately measure exhaust gas components based on the above-mentioned two-step measurement.
Further, since the plurality of layers can be thinly laminated through the printing method, the problem of deterioration in gas penetration can be avoided though the plurality of gas-penetrative solid electrolyte layers are used.
Furthermore, in a case where the multilayer exhaust gas component sensor in accordance with the present invention is applied to a waste incinerating facility or a vehicle, since a very small amount of exhaust gas components such as NOX and HC can be directly measured, detailed combustion control to reduce the exhaust gas components can be performed.